Photographic elements having protective overcoat layers are well known and a wide variety of different coating compositions have been proposed in the past for use as protective overcoats. Such overcoats serve a number of different purposes, such as to provide protection against fingerprints, abrasion and scratching, to protect against water spotting, to provide a particular surface texture such as a matte surface, to provide protection against blocking, and to act as anti-reflection layers which reduce glare. Layers of a temporary nature which are intended to be removed after they have served their purpose and layers which are permanently bonded to the photographic element have been described in the prior art. Protective overcoats can be applied to photographic elements by coating solutions or dispersions of film-forming agents in organic solvents such as described, for example, in U.S. Pat. Nos. 2,259,009; 2,331,746; 2,706,686; 3,113,867; 3,190,197 and 3,415,670; by coating of aqueous film-forming compositions such as are described, for example, in U.S. Pat. Nos. 2,173,480; 2,798,004; 3,502,501 and 3,733,293; by coating of compositions containing discrete, transparent, solid particles of submicroscopic size as described in U.S. Pat. No. 2,536,764; by coating of plasticized polymer compositions as described in U.S. Pat. No. 3,443,946; by coating of polymerized perfluorinated olefins as described in U.S. Pat. No. 3,617,354; by lamination of a protective layer as described, for example, in U.S. Pat. Nos. 3,397,980 and 3,697,277; by radiation-curing of a composition comprising an acrylated urethane, an aliphatic ethylenically unsaturated carboxylic acid and a multi-functional acrylate as described in U.S. Pat. Nos. 4,092,173 and 4,171,979; by radiation-curing of a composition comprising an acrylated urethane, an aliphatic ethylenically unsaturated carboxylic acid, a multi-functional acrylate and a siloxy-containing polycarbinol as described in U.S. Pat. No. 4,333,998; and by radiation-curing of a composition comprising a polymerizable epoxy compound, a cationic initiator, a polymerizable acrylic compound, a haloalkylated aromatic ketone and a polymerizable organofunctional silane as described in U.S. Pat. No. 4,426,431.
Typically, protective overcoats for photographic elements are not required to possess fire-retardant properties. However, certain well known photographic products are intended primarily for large displays. These products include opaque, front-lit materials and transparent or translucent materials which are intended for back-lit applications. Each of these products employs a polymeric support, such as, for example, a support formed from polyethylene terephthalate. When used as back-drops on movie sets, or as free-hanging murals at trade shows, etc, there is a critical need to reduce the flammability of these products so as to reduce the risk of fire. Accordingly, there exists in the photographic art a critical need for a coating composition that can be used to form a fire-retardant protective overcoat on a photographic element.
Numerous compositions have been proposed in the prior art as being suitable for application to combustible materials to render them fire-retardant. However, some of these compositions are opaque and thus unsuitable for use with photographic elements, some are applied from toxic or flammable solvents, some produce relatively toxic combustion products, and some interact with conventional photographic images to result in unwanted color shifts.
The use of phosphorus-containing compounds in forming fire-retardant compositions is very well known and, in relation to use with photographic display materials, such compounds are clearly advantageous as compared to the use of halogenated flame retardants which can generate toxic combustion products. It is also well known to incorporate phosphorus-containing compounds in aqueous film-forming coating compositions used to form fire-retardant protective overcoats, and the ability to use an aqueous composition is especially important in the photographic field. A wide variety of aqueous compositions which include phosphorus-containing compounds have been described heretofore in the patent and technical literature. Thus, for example U.S. Pat. No. 3,345,289 describes fire-retardant compositions containing diammonium phosphate and a polymeric thickener such as a copolymer of methyl vinyl ether and maleic anhydride; U.S. Pat. No. 3,513,114 describes fire-retardant compositions containing polyvinyl acetate, a solvent plasticizer, a carbonific and an ammonium polyphosphate; U.S. Pat. No. 3,562,197 describes fire-retardant compositions containing certain water-insoluble ammonium polyphosphates; U.S. Pat. No. 3,955,987 describes fire-retardant compositions containing monoammonium phosphate and/or diammonium phosphate, urea and/or cyanoguanidine, sucrose and titanium dioxide; U.S. Pat. No. 4,009,137 describes fire-retardant compositions containing a film-forming binder, a carbonific, a dispersing aid and an ammonium polyphosphate; U.S. Pat. No. 4,247,435 describes fire-retardant compositions comprising a carbonific, a film-forming binder, a phosphorus-containing material and an organic chelating agent; and U.S. Pat. No. 4,588,510 describes fire-retardant compositions comprising diammonium phosphate, a polyol, a gas-generating agent and a viscosity builder. These previously disclosed coating compositions serve many useful purposes, but none is capable of meeting the critical needs of the photographic art in relation to providing fire protection for large area display materials.
It is toward the objective of providing a fire-retardant protective overcoat, that is especially adapted to meet the critical needs of the photographic art in providing protection for photographic elements, that the present invention is directed.